1. Field of the Invention
The invention relates to an electromechanical relay with semiconductor-assisted switching. The relay, designed for the selection switching of charges on an electrical network, can be used for this purpose on either an AC or a DC electrical network.
2. Discussions the Background
Electromechanical type relays have one or more mechanical displacement electrical contacts coupled to a mobile element of the magnetic circuit of an electromagnet. The electromagnet is controlled by supplying power to its coil which, by producing an induction flux in the magnetic circuit, drives the movement of the mobile element and the closing or opening of the electrical contacts of the relay.
The electrical contact usually comprises a fixed part and a mobile part, each having pads made of material that is a good electrical and thermal conductor. These pads, which are brought into contact when the relay is closed, must have low contact resistance in order to limit heating during the passage of the current.
The selection switching, by an electromechanical relay, of an electrical circuit under load, especially when the circuit is inductive, produces arcs between the contacts when the circuit is opened or closed. This phenomenon is commonly called sparking.
Indeed, when the closing of the relay is activated, the current is set up in the electrical current through the electrical contact, producing one or more electrical arcs due to rebounds between the mobile contact and the fixed contact.
At opening, the contact cuts off the current travelling through the electrical circuit. This again produces arcs between the contacts. This intensity increases with the level of the current to be cut off and the inductive character of the circuit.
These repeated arcs inevitably cause deterioration in the contact in the course of time and reduce the life of the contact.
In certain electromechanical relays, in order to limit the arc between the contact terminals during the selection switching, either a triac or two back-to-front parallel-connected thyristors are parallel mounted on the terminals of the mechanical displacement electrical contact. When the contact is being closed, a control circuit makes the triac conductive slightly before the closing of the contact. When the contact is being opened, this control circuit makes the same triac conductive slightly before the opening of the contact.
In this type of hybrid relay comprising a parallel-connected triac (or thyristors) on the mechanical displacement contact, the operation of making the contact conductive slightly before the switching of the contact makes almost all the electrical current flow into the fired triac (or thyristor). The opening or closing of the contact at this time will be done with a current appreciably lower than the current in the electrical circuit. The effective closing of the contact will cause the powering-off of the triac or the thyristors as they are short-circuited by the closed contact.
While these hybrid relays improve the lifetime of the contacts, they do not totally eliminate the arc at the time of the switching. Furthermore, as a result of the elasticity proper to the fixed and mobile parts of the contact, when the contact closes or opens, there are rebounds between this fixed part and this mobile part. Consequently, the closing or opening of the contact does not happen in a single operation.
During a closing of the contact, rebounds at the time of the impact between the mobile part and the fixed part of the contact produce a sequence of repeated opening and closing operations whose number will depend essentially on the mechanical characteristics of the contact. These repeated contact opening and closing operations could produce repeated operations of firing and powering-off of the triac or thyristors that are parallel-connected to the electrical contact, and repeated arcs between the contacts whose intensity will depend on the level of the current in the electrical circuit and on its impedance. These arcs could have a very high level in the case of the selection switching of a circuit comprising self-inductance or capacitive loads.
The phenomenon is as follows (we shall describe the phenomenon in the case of a triac it being known that the same phenomenon occurs for back-to-front parallel-connected thyristors): when the closing of the relay is ordered, the triac is made conducive by the control circuit slightly before the closing of the contact in order to let electrical current into the triac. At the time of the first contact between the mobile part and the fixed part of the contact, the triac that is parallel-connected to the contact gets powered off since the voltage at this terminal is substantially zero. The triac is in the insulated state. All the electrical current passes at this point in time into the closed electrical contact. A first rebound of the contact occurs, causing the opening of the contact crossed by the totality of the current in the electrical circuit and the appearance of a selection switching arc. During a short instant of opening that follows the rebound of the contact, the voltage of the electrical circuit reappears at the terminals of the controlled triac, and this triac again gets refired and again lets through current from the electrical circuit into the triac. The contact closes again at the end of the first rebound, and powers off the triac which once again becomes insulated, prompting the passage of the electrical current into the contact. In the same way, a new rebound will reproduce a new selection switching arc of the terminals of the contact until the rebounds stop and the contact is definitively closed.
In the case of an AC network, when the contact is closed, these repetitive arcs will have an intensity all the greater as the selection switching is done for a current close to the maximum current of alternation of current.
When there is a command for opening the relay, the triac is activated just before the opening of the contact. The triac is short-circuited by the contact, the voltage at its terminals is substantially zero and it remains powered off. The contact is opened with the nominal current in the contact. This current disappears very swiftly when the voltage at the terminals of the triac becomes sufficient to fire it. However, a very brief arc occurs at the time of opening. A rebound produces repetitive arcs, in a manner similar to what happens at the time of closing.
In order to overcome the drawbacks of the prior art, the invention proposes an electromechanical relay designed to be inserted into an electrical circuit, the relay comprising a mechanical displacement electrical contact, a transistor parallel-connected with the electrical contact, means to command firstly the closing of the contact and the powering-on of the transistor in response to a first control signal and secondly the opening of the contact and the powering-on of the transistor in response to a second control signal, characterized in that the control means comprise means to:
generate, from the first control signal, a mechanical displacement contact closing signal that precedes the closing of the contact, this closure being done for a voltage V at the terminals of the contact that corresponds to the forward direction of the transistor;
generate, from the first control signal, independently of the closing signal, a first signal for powering on the transistor that starts before the closing of the contact and ends after this closing;
generate, from the second control signal, a mechanical displacement contact opening signal that precedes the opening of this contact, this opening being done for a current in the contact corresponding to the forward direction of the transistor;
generate, from the second control signal, independently of the opening signal, a second signal for powering on the transistor that starts before the opening of the contact and ends after this opening.
In a working of the relay according to the invention in a DC network, the transistor is biased constantly in the forward direction so that, during a command for closing or opening the relay, the transistor is powered on some instants before the closing or opening of the contact and the powering on is stopped some instants after the closing or opening of the contact after the end of the rebounds of the contact.
A parallel-connected transistor with the contact of the electromechanical relay according to the invention, when it is powered on in the forward direction, does not get powered off when it is short-circuited by the mechanical displacement contact which has the advantage, as compared with prior art relays using triacs and transistors, of continuing to be conductive during successive openings at the time of the rebounds of the contact. The transistor, which is powered on in the forward direction, totally eliminates the repetitive arcs due to rebounds at each opening of the contact, the current of the electrical circuit instantaneously passing into the transistor.
In one embodiment of the relay according to the invention used in an AC network:
the first signal for powering on the transistor is generated when the voltage V corresponding to the forward direction of the transistor is close to the change in direction of the alternation of the voltage V at its terminals;
the second signal for powering on the transistor is generated when the current corresponding to the forward direction of the transistor is close to the change in direction of the alternation of current in the contact.
In the case of use in an AC network, the fact that the transistor is powered on during a closure of the contact, for a voltage in the forward direction of the transistor that is close to the change in alternation of voltage, namely close to a low voltage as compared with the maximum voltage of the network, means that it is possible to reduce the size of the transistor. Indeed, the current flowing through the transistor during the short period of powering on the transistor (as compared with the period of the AC voltage of the network), will have a low value, the voltage at the terminals of the network being close, at this time, to the change in alternation and therefore having a low value close to zero volts.
In the same way, an opening of the contact for a current in the forward direction close to a change in alternation of current, namely a current close to zero amperes, will mean that the size of the transistor can be reduced.
In the embodiments of the relay according to the invention, the transistor parallel-connected with the electrical contact may be chosen from among the IGBT (insulated gate bipolar transistor) type transistors, bipolar transistors or MOS transistors.
In a variant of the relay according to the invention, the transistor is series-connected with a diode providing protection against reverse voltages at the terminals of the transistors. The protection diode enables the use of the transistor in networks whose voltage is higher than the reverse voltage that can be borne by the transistor, this reverse voltage being borne by the diode.
In one embodiment, the relay according to the invention uses a microcontroller having, firstly, inputs respectively receiving the commands from the relay, a piece of information on current in the electrical circuit and a piece of information on voltage at the terminals of the mechanical displacement electrical contact and, secondly, a control output giving the control signals for opening and closing the contact and an output for powering on the transistor.